Twenty years after the most recent spacecraft encounter with Saturn, another
spacecraft is speeding toward the beautifully ringed planet. And this one
will stay.

When NASA’s Voyager 2 spacecraft sped past Saturn in August 1981, scientists
sought answers to questions raised by two other spacecraft that had visited
Saturn in the previous two years.

That was 20 years ago. No spacecraft has reached Saturn since then, but one
is now on its way there.

The Cassini-Huygens mission of NASA and the European Space Agency will reach
Saturn on July 1, 2004. Cassini will become the first craft to orbit Saturn,
and a half year after arrival, its piggybacked Huygens probe will descend
onto Saturn’s moon Titan. Scientists will use them to answer questions
remaining from the earlier explorations.

Dr. Ellis Miner, an astronomer at the Jet Propulsion Laboratory since 1965,
has worked on missions to Saturn and six other planets. “Whenever we
investigate a planet in detail,” he says, “we discover answers to a lot of
questions, but we inevitably raise even more new questions.”

Saturn’s First Close-Ups

Miner was assistant project scientist for Project Voyager as Voyager 2
approached Saturn 20 years ago. When that spacecraft was launched in 1977,
no spacecraft had visited Saturn. However, while Voyager 2 was on its way
toward Saturn, Pioneer 11 got the first close look at the spectacularly
ringed planet, then Voyager 1 studied it in more detail. So, scientists
learned much about the Saturn system by the time Voyager 2 got there. They
learned Saturn had more moons, more rings, more heat and faster winds than
anyone had known. They discovered that its biggest moon, Titan, hides
beneath a thick, nitrogen-rich atmosphere resembling the early Earth’s.

“We made several adjustments to the planned observations by Voyager 2 based
on what we had seen with Voyager 1,” Miner recalls. “For example, we
discovered some moons with the Voyager 1 imagery, and we wanted to make sure
we got some new observations of those moons to refine their orbits and see
if we could learn something about their sizes and surface characteristics as
we flew by.”

Flight engineers programmed Voyager 2 to get a better look at one of
Saturn’s middle-sized inner moons, Enceladus, after Voyager 1 images showed
that Enceladus had an interestingly bright surface. “We got those closer-up
pictures of Enceladus showing a surface that had obviously melted in the
geologically recent past,” says Miner. Tidal forces on Enceladus from
Saturn’s gravity may be warming that moon’s interior the way the tidal tug
from Jupiter heats the Jovian moons Io and Europa.

The assortment of follow-up observations attempted with Voyager 2 probably
contributed to a temporary jam in movement of the platform on which the
spacecraft’s camera is mounted. “We found so many things to do with Voyager
2 after the Voyager 1 flyby, that may have led to the scan-platform problem,
because we had the thing swinging back and forth in the sky so fast and so
furiously that we think that’s what drove the lubricant out of the gears and
caused the gear train to seize,” Miner says.

The problem prevented picture-taking temporarily after the spacecraft’s
closest approach to Saturn. A few days later, engineers sent up commands to
try a low-speed turn for a view back toward Saturn that had not been
possible with Voyager 1 because of its trajectory. Scientists anxiously
watched monitors in the operations center at JPL to see if the maneuver
would work.

“It was a few seconds later than we anticipated when something finally
showed up on the screen, but at first none of us could make out what it
was,” Miner says. “Then it dawned on me that looking back in that direction,
Saturn was upside down in the monitor because of the direction things were
pointed, so I ran up to the screen and said, ‘Look! Here’s the ring, and
here’s Saturn, and here’s a shadow.'”

The look-back pictures showed the rings as seen from the unlit side, with
sunlight coming through them, revealing additional information about the
particles making up the rings. The scan platform worked for taking pictures
of the outer moon Phoebe as Voyager 2 headed out of the Saturn system, and
for the spacecraft’s later flybys of Uranus and Neptune.

During a press conference at the end of Voyager 2’s Saturn flyby, a reporter
asked JPL’s Dr. Ed Stone, project scientist for the mission, what percentage
of the expected science had been obtained during the flyby. “Ed’s answer was
a classic,” Miner recalls, “He said, ‘Two hundred percent.’ And it’s true.
We got a lot more than we anticipated despite that seizure of the scan
platform. I still look back on it, though, and wish we had been able to get
those close-up pictures of Tethys [the next moon out from Enceladus] and the
other things we had planned in that time period when the platform was

The Next Explorer: Cassini

Miner began working on the Cassini-Huygens mission to Saturn in 1990,
shortly after Voyager 2 completed its Neptune flyby. This mission has three
main differences from the earlier Saturn explorers that will enable it to
answer many questions they left open, he explains: “One, we have a probe so
we can study Titan in place. Two, it’s an orbital mission so we can watch
things happen over a long period of time. And three, the complement of
instruments on Cassini is both more extensive and far more sophisticated
than the one we had on Voyager.”

A few of the questions for Cassini and Huygens are: Does Titan have lakes of
liquid hydrocarbons? What happens in its atmosphere? Do Saturn’s winds and
rings change much in the span of a few years? How do Saturn’s moons and
magnetic field affect the rings? Does Enceladus have active ice volcanoes?

Cassini was launched four years ago. It passed Jupiter last December. Twenty
years after Saturn’s most recent visit from Earth, the next emissary is on
the last leg of its journey and has orders to stick around once it arrives.